84
Fundamentals of Corrosion
NO x relative to SO 2 (in equivalents) has increased markedly, which stresses
the importance of NO x emissions.
In combustion processes, most of the nitrogen oxides are emitted as NO.
4.2.2.4 Chlorides
Chlorides are deposited mainly in the marine atmosphere as droplets or as
crystals formed by the evaporation of spray carried by the wind from the
seas. Other sources of chloride emission are coal-burning and municipal
incinerators. Most coals have a chloride content of 0.09 to 0.15%. In highchlorine coals, values of 0.7% are found. In the burning of coal, most of the
chlorine is emitted as gaseous HCl.
4.2.2.5 CO 2
Carbon dioxide (CO 2 ) occurs in the atmosphere in a concentration of 0.03 to
0.05% by volume, varying slightly with the time of day and the season of the
year due to its cycle in nature.
4.2.2.6 Concentration of Different Species
The concentrations of the various species in the electrolyte on the surface
vary greatly with respect to such parameters as deposition rates, corrosion
rate, intervals between rain washings, the presence of rain shelter, and drying conditions.
It would be expected that the concentration in the electrolyte film will be
low during a rainy period, while a highly concentrated solution may form
after a long period without washing.
The pH of the water film is difficult to specify. A moisture film in contact
with an atmosphere highly polluted with SO x may initially have a pH value
as low as 2. Due to the acid rain or fog, the moisture film may also have a low
pH value. Because of reaction with the metal and the corrosion products, the
pH value will usually increase. When a steady state has been reached, the pH
is generally on the order of 5 to 6.
4.2.3 Temperature
The overall effect of temperature on corrosion rates is complex. During
long-term exposure in a temperate climatic zone, the temperature appears
to have little or no effect on the corrosion rate. As the temperature increases,
the rate of corrosive attack will increase as a result of an increase in the
rate of electrochemical and chemical reactions as well as the diffusion rate.
Consequently, under constant humidity conditions, a temperature increase
will promote corrosion; conversely, an increase in temperature can cause a
decrease in the corrosion rate by causing a more rapid evaporation of the
Fundamentals of Corrosion
NO x relative to SO 2 (in equivalents) has increased markedly, which stresses
the importance of NO x emissions.
In combustion processes, most of the nitrogen oxides are emitted as NO.
4.2.2.4 Chlorides
Chlorides are deposited mainly in the marine atmosphere as droplets or as
crystals formed by the evaporation of spray carried by the wind from the
seas. Other sources of chloride emission are coal-burning and municipal
incinerators. Most coals have a chloride content of 0.09 to 0.15%. In highchlorine coals, values of 0.7% are found. In the burning of coal, most of the
chlorine is emitted as gaseous HCl.
4.2.2.5 CO 2
Carbon dioxide (CO 2 ) occurs in the atmosphere in a concentration of 0.03 to
0.05% by volume, varying slightly with the time of day and the season of the
year due to its cycle in nature.
4.2.2.6 Concentration of Different Species
The concentrations of the various species in the electrolyte on the surface
vary greatly with respect to such parameters as deposition rates, corrosion
rate, intervals between rain washings, the presence of rain shelter, and drying conditions.
It would be expected that the concentration in the electrolyte film will be
low during a rainy period, while a highly concentrated solution may form
after a long period without washing.
The pH of the water film is difficult to specify. A moisture film in contact
with an atmosphere highly polluted with SO x may initially have a pH value
as low as 2. Due to the acid rain or fog, the moisture film may also have a low
pH value. Because of reaction with the metal and the corrosion products, the
pH value will usually increase. When a steady state has been reached, the pH
is generally on the order of 5 to 6.
4.2.3 Temperature
The overall effect of temperature on corrosion rates is complex. During
long-term exposure in a temperate climatic zone, the temperature appears
to have little or no effect on the corrosion rate. As the temperature increases,
the rate of corrosive attack will increase as a result of an increase in the
rate of electrochemical and chemical reactions as well as the diffusion rate.
Consequently, under constant humidity conditions, a temperature increase
will promote corrosion; conversely, an increase in temperature can cause a
decrease in the corrosion rate by causing a more rapid evaporation of the